1. Field of the Invention
The present invention relates to a charger for lithium ion secondary battery, and a method of charging the same.
2. Description of the Related Art
Lithium ion secondary batteries are generally charged based on constant-current/constant-voltage charging at 4.2 V per a single cell, as indicated by curves “a” and curves “b” in FIG. 4 and FIG. 5. In the charging based on combination of constant-current charging and constant-voltage charging, the lithium ion secondary battery is charged under constant current until the cell voltage reaches a set voltage Vc, and then under constant voltage, changed over from constant current, after the cell voltage reaches the set voltage Vc.
Known methods detecting an end point of charging of the lithium ion secondary battery include a current detection system shown in FIG. 4, in which lowering in the charging current caused by the constant-voltage charging in the constant-voltage charging period after the constant-current charging, is monitored, and the charging is terminated when a predetermined completion current I0 is reached; and a ΔV detection system (see Japanese Patent Application Publication No. H10-32938) shown in FIG. 5, in which the charging period and the opening period are repeated at regular intervals, wherein voltage difference ΔV between a specified voltage (for example, upper limit voltage Vh of the lithium ion secondary battery) and the open circuit voltage Vt in the opening period is calculated, and the charging is terminated when the voltage difference ΔV falls in a range not exceeding a predetermined completion difference voltage ΔV0.
Detection of the end point of charging of the lithium ion secondary battery requires charging under constant voltage, but the constant-voltage charging was disadvantageous in that the amount of charged energy decreased due to decrease in the charging current, so that the charging time was elongated.
One possible measure may be such as elevating the set voltage Vc where the constant-current charging and the constant-voltage charging are changed over, thereby elongating the charging time under constant current, and thereby shortening the charging time required up to completion of the charging. The elevation of the set voltage Vc, however, results in over-voltage in the constant-voltage charging period, because voltage loss is generated due to circuit impedance between the constant-current/constant-voltage control circuit and a charging terminal, and is therefore causative of degradation of the lithium ion secondary battery.
In general, there are upper limit values specified for the voltage value and the current value for the charging of lithium ion secondary batteries, and exceeding of these values may induce degradation of the lithium ion secondary batteries, raising a need of completing the charging within the specified upper limits.